1. Field of the Invention
The present invention relates to telephone sets fitted with a speech signal loud-speaker also serving as a ringer and more particularly to a device for regulating the power provided to the loud-speaker during a ring signal so that the sound power is at a maximum level as a function of the power available on the telephone line.
2. Discussion of the Related Art
FIG. 1 schematically shows some elements of a conventional telephone system. A telephone exchange 5 is connected to terminals A and B of a telephone set through a two-wire line L having an impedance Z.sub.L. Terminals A and B are connected to the input of a rectifying bridge 10 which provides a voltage V.sub.2. Voltage V.sub.2 is filtered by a capacitor Cf and is applied to the input of a DC/DC converter 12 which provides at a terminal U a stabilized voltage U. The input and output of converter 12 have a common ground G. Voltage U, generally approximately 5 volts, is intended to supply various circuits of the telephone set and particularly the ringing circuit. At the input of converter 12 is shown an apparent impedance Zp.sup.* equal to the mean impedance of the ringing circuit, as seen from the input of converter 12.
The ringing circuit includes an amplifier 14 supplied by voltage U and powering a loud-speaker LS through a coupling capacitor Cc. Amplifier 14 has a gain K higher than 1 because it serves, outside ringing periods, to provide to loud-speaker LS a low-level speech signal, with a sufficiently high power.
A microprocessor 16 is also supplied by voltage U. When a ringing signal is detected on line L, microprocessor 16 provides a square wave signal F having its frequency modulated to generate a ringing melody to be reproduced by loud-speaker LS. The peak-to-peak amplitude of signal F is equal to U. In order not to saturate amplifier 14, signal F is divided by K in an attenuator 17 before it is applied to amplifier 14. Thus, at the output of amplifier 14, a signal practically equal to signal F is again provided. Due to the coupling capacitor Cc, the voltage across loud-speaker LS is a square wave voltage oscillating between values -U/2 and U/2. Under these conditions, the power provided to the loud-speaker is equal to a maximum value P.sub.M.
The ringing signal provided by the telephone exchange 5 to the telephone line is assumed to be sufficient to supply the ringing circuit. The ringing signal is an a.c. voltage having an r.m.s. value V.sub.1 variable about a nominal value (80 volts in France).
The higher the line impedance Z.sub.L (the longer the line L) and the lower value V.sub.1, the smaller the available power to supply the ringing circuit. Assuming converter 12 is perfect, the power Pc consumed by the ringing circuit is equal to the power dissipated in impedance Zp.sup.*. This consumed power can be rendered maximum by suitably selecting impedance Zp.sup.*, that is, Zp.sup.* =Z.sub.L. This maximum power is defined as the available power Pd, expressed by: EQU Pd=V.sub.1.sup.2 /4Z.sub.L ( 1)
For the sake of simplification, it will be assumed that the circuits which consume all the power (Pc) are the loud-speaker and the amplifier.
FIG. 2 shows a curve in dashed lines of the variation of the available power Pd as a function of the line impedance Z.sub.L. The solid line represents the consumed power Pc. The consumed power Pc is constant and equal to the maximum value P.sub.M for values of impedance Z.sub.L ranging from 0 to an impedance Z.sub.L where the available power Pd is equal to power P.sub.M. Power Pc is constant within this range because the ringing circuit has a constant impedance supplied by a constant voltage U. When the line impedance Z.sub.L becomes higher than Z.sub.1, the available power Pd becomes lower than the maximum power P.sub.M always needed by the ringing circuit. Then, converter 12 is no longer suitably supplied, its output voltage U drops and the ringing circuits misfunction or may no longer operate at all.
In the prior art, to avoid a drop of voltage U when impedance Z.sub.L becomes higher than Z.sub.1, it has been proposed that amplifier 14 has a variable gain K. Gain K is regulated so as to decrease the power dissipated in the loudspeaker when the consumed power Pc approaches the available power Pd, so that the consumed power Pc remains lower than the available power Pd.
In practice, gain K is regulated proportionally to the input voltage V.sub.2 that varies in the same way as the available power Pd. The ratio between gain K and voltage V.sub.2 is adjusted so that the consumed power Pc is equal to the available power Pd under nominal conditions, i.e., when the line impedance Z.sub.L reaches a nominal value Z.sub.0 (2000 .OMEGA. in France). The available power Pd under nominal conditions is referenced P.sub.0. With this configuration, when impedance Z.sub.L varies about Z.sub.0, the consumed power Pc linearly varies, as represented in dot and dash lines, tangentially to curve Pd at point (Z.sub.0,P.sub.0) and remains lower than the available power Pd. When the line impedance Z.sub.L reaches a value Z1' lower than value Z.sub.1, gain K reaches its nominal value where the peak-to-peak amplitude of the voltage across the loud-speaker is equal to value U of the supply voltage. At this moment, even if gain K further increases, the voltage provided to the loud-speaker can no longer increase and the consumed power remains equal to the maximum power P.sub.M.
However, an amplifier, supplied with a voltage U and providing a rectangular voltage having an amplitude lower than U to a load has an efficiency lower than 1. The higher impedance Z.sub.L becomes with respect to Z.sub.1 ', the more the efficiency decreases, and the more amplifier 14 dissipates power to the detriment of the loud-speaker. Thus, for lines having impedance approaching the maximum tolerable value (3500 .OMEGA. in France), the power supplied to the loud-speaker may be insufficient although the consumed power Pc is not negligible because of power dissipation in the amplifier and not in the loud-speaker.